1. Field of the Invention
The present invention relates to an oxygen sensor for use in a system, for an internal combustion engine, such as an air fuel ratio control system, and a method for producing such an oxygen sensor.
2. Description of Related Art
In a known air fuel ratio control system for an internal combustion engine, an oxygen sensor is arranged in an exhaust system of the internal combustion engine to detect an air fuel ratio and combustion control is done in accordance with the air fuel ratio detected by the oxygen sensor, so that an increase in a purification efficiency of an exhaust gas is obtained in a three way catalytic converter arranged in an exhaust system of the internal combustion engine.
Such an oxygen sensor includes a detecting element (an oxygen sensor element), which is constructed as a body of a solid electrolyte having oxygen ion conductivity. Namely, the solid electrolyte body is formed as a cup shape, in which an air chamber is formed. Furthermore, the solid electrolyte body forms, at its outer surface, an outer electrode and, at its inner surface, an inner electrode, which is in contact with the air in the air chamber. Furthermore, inside the air chamber, a heater is arranged so that the oxygen sensor element is heated to an activating temperature.
In the prior art structure of the oxygen sensor, as shown in FIG. 16, a heater 9 as rod shape of a circular cross sectional shape is used. The heater 9 is constructed by a heat generating part 21 in which a heat generating element is housed and a supporting section 22 for supporting the heat generating part 21 and for housing lead members 22 electrically connected to the heat generating element in the heat generating part 21.
Now, a method for producing the above construction of the heater 9 will be explained. Namely, as shown in FIG. 17, a ceramic green sheet 90 is subjected to coating with a layer of a desired pattern 200 to be formed into a heat generating element 210 and lead elements 220 by being subjected to a later firing process. Then, on the sheet, a organic binder, which is an ethyl cellulose dissolved in an organic solvent, is painted. Then, the sheet 90 is wrapped around a core rod 900 made of a ceramic material. Then, the core rod 900 together with the sheet 90 wrapped around the rod 900 is subjected to a firing in a furnace of a high temperature in a range between 1400 to 1500.degree. C. Then, at an end of the heater 9, terminal elements 290 are formed so that they are in an electric connection with the lead element 220. Then, lead wires 29 are connected to the terminal elements 290, respectively, by means of a soldering using a soldering material such as AuCu in a vacuum condition at a high temperature in a range between 950 to 1000.degree. C. Finally, the lead wires 29 are connected to an outside electric power source, so that the heat generating element 210 is fed with electric power via the electrode element 220.
In the above mentioned structure in the prior art in FIG. 16, the heater 9 is formed as the rod of a circular cross sectional shape, which, on one hand, causes the value of the cross sectional area to be increased and, on the other hand, causes a loss of the heat to be increased in the upward direction. As a result, the prior art structure of the rod of the circular cross sectional shape is defective in that the speed of the increase in the temperature of the oxygen sensor element is reduced. Furthermore, the circular cross sectional shape of the rod causes the clearance to be small with respect to the faced inner wall of the air chamber, which results in a reduced amount of air introduced into the air chamber, which causes the precision to be reduced in the detection of the oxygen density by the oxygen sensor.
Furthermore, according to the method for producing the heater 9 as briefly explained with reference to FIG. 17, a production efficiency is low due to the fact that a formation of the heater pattern is done on a rod by rod basis.